Using a swf runtime super cool "Drawing & Charts" exploiting the advantaqge to positioning the SWF component inside DOM using javascript with possibility to pass parameters. Its capability is not restricted rather increased with the ability of scalable vector drawing.

The Ext.Data package has been overhauled with interesting additions.

Its not last but the UI which matters the most is now supported by CSS3 themes, and the theme has been overhauled to give more richness with reduced size on browser.

Last but not the lease the "Performance" aspect has been improved drastically.
The reduced size load at DOM(i.e. DOM reduction) with an implied and tuned effect on speed gains. And the most interesting is the Infinite Scroll on Grids with data request on demand to scroll.